It is proposed to carry out continuous cultivation of T. pallidum in cultures of mammalian tissues. Preliminary evidence indicates that we may be observing limited multiplication. Efforts are being devoted to improvement of cultural conditions in order to demonstrate unequivocal replication of T. pallidum. Deoxyribonucleic acid (DNA) assays will be carried out to determine if simultaneous increases occur concomitantly with apparent increases in treponemal numbers. Should we be able to accomplish continuous multiplication of T. pallidum in vitro the organisms will be tested for virulence in rabbits. The isolate will also be investigated for its genetic relationship to the rabbit-passaged Nichols strain of T. pallidum from which it originated.